The invention relates to the general field of telecommunications. The invention relates in particular to a medium access control (MAC) protocol.
In a telecommunications network in which a plurality of nodes share a common communications medium, simultaneous transmissions can interfere, causing collisions and loss of packets. This applies in particular to wireless networks such as wireless mesh networks, WiFi local networks, or lossy low power networks (LLNs).
Numerous medium access control networks have been devised for coping with this problem. In particular, the IEEE-802.11 family of protocols relates to WiFi local networks and the IEEE-802.15.4 family of protocols relates to wireless personal networks.
The development of networks of wireless sensors has introduced a new constraint in the design of a MAC protocol, namely efficiency in terms of energy consumption. Thus, so-called “low-power listening” (LPL) MAC protocols have been designed. In that type of protocol, the nodes sleep for long periods, and a transmitter/receiver node pair may be selected to communicate during time of wakefulness. The mechanisms proposed by those protocols for avoiding collisions may be classified in two categories: deterministic mechanisms and probabilistic mechanisms.
Deterministic mechanisms are based on a pre-established plan in which each logical channel is dedicated to a specific node. Such mechanisms adapt poorly to unpredictable traffic or they require the plan to be revised frequently since that leads to large energy expenditure. In addition, extensibility is poorly ensured since any one particular node can use only the channel that is allocated thereto.
Probabilistic mechanisms are based mainly on a carrier sense multiple access (CSMA) mechanism. In particular, several protocols use knowledge about time that is shared between the nodes to act during a fixed contention window to determine which node can transmit during the following time of wakefulness.
For example, the document by K. Jamieson, H. Balakrishnan, and Y. C. Tay, “Sift: a MAC protocol for event-driven wireless sensor networks”, published in EWSN (K. Römer, N. Karl, and F. Mattern, eds.), Vol. 3868 of Lecture Notes in Computer Science, pp. 260-275, Springer, 2006, describes a mechanism in which a period of transmission is preceded by a contention window that is subdivided into CW time slots. Each node that has data for transmission selects at random one particular slot r among the CW time slots. Thereafter, during the slots preceding the slot r, the node listens and skips its turn if it hears a transmission. If during the slot r, the node does not skip its turn, then it transmits a signal to mark the time slot r and it verifies that no collision has taken place. At the end of the contention window, the node that was able to transmit during the time slot it selected determines that it can transmit data during the following transmission period.
That solution allows the sole node that is allowed to transmit during the next allocation of the medium to be determined, and for this to be done locally, i.e. on the basis solely of information available at each node. Furthermore, the dimensioning of the mechanism (i.e. the number of slots in the contention window and the probability function used for selecting a slot randomly) can depend on traffic load alone and not on network properties. Finally, that solution makes it possible to adapt to traffic that is varying, including in bursts.
Nevertheless, utilization of the available bandwidth remains limited. If the selected node has little data to transit relative to the capacity of the medium during the allocation duration, then the capacity is under-utilized, since no other node can access the medium during that duration.
The same problem is to be found in document US 2008/0219286, which describes a collision-avoidance mechanism in which a single station is selected to transmit in a second time interval as a function of whether or not contention messages are received in a first time interval.